Sub-Band Filling and Hole Transport in Polythiophene-Based Electrolyte-Gated Transistors: Effect of Side-Chain Length and Density

The relationship between hole density and conductivity in electrochemically gated polythiophene films is examined. The films are integrated into electrolyte-gated transistors (EGTs), so that hole accumulations can be electrochemically modulated up to ≈0.4 holes per thiophene ring (hpr). Polythiophenes include poly(3-alkylthiophenes) (P3ATs) with four different side chain lengths – butyl (P3BT), hexyl (P3HT), octyl (P3OT), or decyl (P3DT) – and poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) and poly(3,3′′′-didodecyl[2,2′:5′,2′′:5′′,2′′′-quaterthiophene]-5,5′′′-diyl) (PQT). Analysis of the drain current – gate voltage (I_D–V_G) and gate current – gate voltage (I_G–V_G) characteristics of the EGTs reveals that all six polythiophene semiconductors exhibited reversible conductivity peaks at 0.12 – 0.15 hpr. Conductivity is suppressed beyond ≈0.4 hpr.The maximum carrier mobilities of the P3AT semiconductors increase, and hysteresis of the conductivity peaks decreases, with increasing alkyl side-chain length. PBTTT and PQT with reduced side chain densities exhibit the largest hysteresis but have higher hole mobilities. The results suggest that at ≈0.4 hpr, a polaronic sub-band is filled in all cases. Filling of the sub-band correlates with a collapse in the hole mobility. The side-chain dependence of the peak conductivity and hysteresis further suggests that Coulombic ion-carrier interactions are important in these systems. Tailoring ion-carrier correlations is likely important for further improvements in transport properties of electrochemically doped polythiophenes.